Glade instantiation: Simplify, make more robust, and test.
* glom/glade_utils.h: get_glade_widget_derived_with_warning(): Don'te an ID. This requires the class to have a static glade_id member and glade_developer (bool) member, telling us what file and what ID to use. This associates the IDs and filenames with the class, instead of being in other files, to avoid duplication and to avoid using the wrong ones. This allows us to remove get_glade_developer_widget_derived_with_warning(). * Many files: Use these new methods, to avoid mentioning glade IDs. * tests/test_glade_derived_instantiation.cc: Try to instantiate all the known derived glade dialogs.